1. Technical Field
This disclosure relates generally to image sensors, and more particularly but not exclusively, relates to complementary metal-oxide-semiconductor (“CMOS”) image sensors.
2. Background Art
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, for example, CMOS image sensors (“CIS”), has continued to advance at a great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
FIG. 1A illustrates a conventional front side illuminated CIS 100. The front side of CIS 100 is the side of a substrate 105 of a semiconductor material upon which pixel components are disposed and over which a metal stack 110 for redistributing signals is formed. Metal layers (e.g., metal layer M1 and M2) of metal stack 110 are patterned in such a manner as to create an optical passage through which light incident on the front side of CIS 100 can reach a photosensitive or photodiode (“PD”) region 115 in substrate 105. To implement a color CIS, the front side further includes a color filter layer 120 disposed under a micro-lens 125. Micro-lens 125 aids in focusing the light onto PD region 115.
CIS 100 includes pixel circuitry 130 disposed adjacent to PD region 115. Pixel circuitry 130 provides a variety of functionality for regular operation of CIS 100. For example, pixel circuitry 130 may include circuitry to commence acquisition of an image charge within PD region 115, to reset the image charge accumulated within PD region 115 to ready CIS 100 for the next image, or to transfer out the image data acquired by CIS 100.
FIG. 1B is a plan view of eight neighboring CIS 100 pixels within a CIS array 140. As illustrated in FIG. 1B, micro-lenses 125 of adjacent image sensors in CIS array 140 are separated by gaps 145 between the pixels—for example, where each image sensor is designed for sensing light of a particular wavelength (e.g. one of red, green, blue). Typically, conventional CIS arrays have several associated problems. For example, performance of CIS array 140 can be reduced by the reflection of incident light at the interface between the air and micro-lens 125. Gaps 145 between micro-lenses 125 can reduce fill factor, i.e. the portion of the area of CIS array which actually captures light. After formation, micro-lenses 125 can be damaged in subsequent CIS fabrication processes, such as cleaning operations. Moreover, micro-lenses 125 tend to outgas and contaminate subsequent fabrication processes, such as to the dam layer supporting the cover glass over the micro-lens array.
FIG. 1C illustrates inclusion of a conventional coating 150 to CIS array 140 to mitigate some of the above-described problems. Because of the refractive index difference between air and micro-lens 125, when light shines on image sensors, certain amount of the illumination is reflected at the surface of micro-lens 125. As it is hard to change the micro-lens material, reducing light reflection may be achieved by adding between the air and the micro-lens a coating layer 150 whose refractive index is between that of air and that of the micro-lens. To date, traditional coating application techniques have not succeeded in image sensor applications. For example, coating 150 extends non-conformally over gaps 145 between micro-lenses 125—e.g. where coating 150 fails to conform to the profile of micro-lens 125 and instead fills gaps 145 with a “U-shape” profile. Such non-conformal coating of an image sensor micro-lens tends to degrade image sensing performance.